At present, a very large part of the coal reserves in the United States is found in seams which are very difficult to mine because of physical constraints and economical considerations. Approximately one trillion tons of coal are found in seams 14-28 inches thick; however, less than five percent of all U.S. coal production comes from these resources. In the coal rich region of Appalachia, approximately forty percent of the minable coal reserves exist in hard to get 28-42 inch thick seams. Still other mineral deposits in seams greater than 42 inches in thickness also remain untapped because of their inaccessibility.
In the past, these mineral reserves were only partially exploited by methods such as contour mining and highwall auger mining along the mountainous ridges where the coal seam outcropped and was easy to reach. This type of contour strip mining removed only a small fraction of the total coal reserve, and further scarred the landscape with highwalls that stand today as steep, unstable and unsightly hillside cuts. By 1977, it was determined that strip mining had left about 25,000 miles of unreclaimed highwalls in various regions of the United States. These previous mining efforts have thus not only seriously impacted the environment, but have left the huge coal reserve in relatively thin seams largely untapped. A need therefore exists for improved and cost effective mining technology to address the problem of recovering seams of mineral deposits, particularly in mountaintop ridges which have previously been contour-strip mined.
In the patent arts, various machines and methods have been disclosed for mining coal along the length of an elongated coal face. In U.S. Pat. No. 4,225,186 (Stratton) there is disclosed an apparatus for mining the coal seams in which a cutting tool extends from a service vehicle and cuts a deep slot about 24 feet in length in one half of the earth formation mine. This cutting tool is adjustable in terms of elevation, sweep and pitch and is designed for one-way movement through the coal seam. The other half of the earth formation mine is completed by a second sweep of this vehicle. This system requires a complex elevator assembly needed to adjust the cutting tool and cut deep slots in the coal seam, and can potentially disturb the overburden or cause massive overburden fall. The length of the cutting tool is limited to a maximum of about 24 feet because of the cantilever reaction forces on the bar and the base machine, and therefore this system is limited to areas where access to the seam is provided every 48 feet. There thus still exists a need for a system for extracting a mineral seam from a large overburden (such as mountain ridges over 1,000 feet in width) in a manner which eliminates the need for roof supports yet which does not induce massive fall of the overburden.
Other systems for mining coal are also known, such as those disclosed in U.S. Pat. Nos. 4,118,072 (Kelley et al), 1,821,439 (Levin), 4,445,723 (McQuade), and 4,278,293 (Paurat et al), but these are not directed specifically to the mining of coal deposits in relatively thin seam formations. Further, these previous systems are commonly subject to cave effects and roof control problems which can trap or damage the machines and severely disrupt mining efforts. What is desired therefore, is a simple, cost effective, efficient method for mining coal in seams of varying thickness which can be accomplished safely, which does not require miners underground, which eliminates the need for roof supports, and which can be carried out without the risk of undesirable massive falls of overburden.